-LRB- CNN -RRB- -- Help trickled in Thursday , three days after a magnitude-7 .7 earthquake struck off Indonesia , triggering a tsunami that has killed at least 311 people and left more than 400 missing .

An assessment team from the nonprofit group SurfAid International had reached the disaster zone , but the area 's remoteness prevented further contact with headquarters in Sumatera Barat , Indonesia .

SurfAid had shipped 1,000 shelter kits to victims , and is preparing to send more supplies once communication is established , spokeswoman Jossi Syahrial said Thursday .

The group was trying to work with government officials to establish an information and coordination hub for all non-governmental organizations , she said .

The hard-hit Mentawai Islands region is among Indonesia 's myriad islands .

`` It 's very difficult '' to reach the region either by boat or road , said Andrew Judge , CEO of SurfAid .

The trip takes at least 10 hours in good conditions , according to aid agencies .

The Indonesian Red Cross also had sent at least two aid teams . One aid shipment included about 400 body bags .

Urgent needs are expected to include clean water , food , blankets , clothing , medical attention , emergency shelter and hygiene kits , said Ita Balanda , a program manager for the World Vision aid organization in Padang , Indonesia .

An estimated 7,900 households were affected , she said .

`` The worst hit area is North and South Pagai Island . Reports of villages flattened are coming from there , '' said Antorizon of the West Sumatra disaster management agency , who like many Indonesians goes by one name .

The first pictures from North Pagai Island showed damaged and flattened structures .

Monday 's quake generated a `` significant '' tsunami , the Pacific Tsunami Warning Center said . Some of the missing might include people who are unaccounted for after fleeing to higher ground , said Henri Dori Satoko , head of the Mentawai Islands parliament .

Though communication poses a challenge in the remote area , some witnesses in West Sumatra reported seeing a wave 6 meters -LRB- nearly 20 feet -RRB- high . Other reports described the tsunami as being about 3 meters -LRB- almost 10 feet -RRB- high .

At least one village with a population of about 200 people was swept away , with only 40 people recovered , Satoko said .

The quake struck at 9:42 p.m. Monday , triggering a tsunami warning . Its epicenter was 240 kilometers -LRB- 150 miles -RRB- south of Padang , at a depth of 20.6 kilometers -LRB- 12.8 miles -RRB- , according to the U.S. Geological Survey . The magnitude was revised upward from a preliminary magnitude of 7.5 .

`` On Mentawai , a -LSB- tsunami -RSB- warning was issued seven minutes after detection of earthquake , which is what the existing system is capable of doing . Preliminary report indicates the destructive waves hit the Mentawai Islands after five minutes . ... For Mentawai villagers , the warning is late , as they are very close to the tsunami source , '' said Sanny Jegillos of the U.N. Development Programme . He oversees the Indian Ocean tsunami warning system , including Indonesia .

Indonesia 's government has since warned residents of West Sumatra province to stay alert and stay away from the coastline for up to five days because of the possibility of more earthquakes , Balanda said .

The city of Padang and the Mentawai Islands are at the meeting place of two tectonic plates , making them vulnerable to earthquakes and tsunamis .

On December 26 , 2004 , a 9.1-magnitude earthquake struck off northern Sumatra . A tsunami generated by that earthquake killed more than 225,000 people in 14 countries -- mainly India , Indonesia , Maldives , Sri Lanka and Thailand . The Indonesian region of Banda Aceh was hard-hit : About 150,000 died there .

CNN 's Kathy Quiano and Stan Grant contributed to this report .

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The area 's remoteness has limited help and communications

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At least 311 people have died and more than 400 are missing

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A team from SurfAid International has reached the area

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The nonprofit group also has shipped 1,000 shelter kits